Fabrication of nanoelectronic devices using self-assembled two-dimensional arrays of monolayer protected clusters

The successful endeavor of the semiconductor industry to double the density of devices every 18 months (i.e. Moore\u27s law) has relied on the ability to continually reduce feature sizes. The ability to design devices with features smaller than a few nm (10−9 m) will require fabrication techniques and interconnect architectures that are unfeasible using lithography. This necessitates a paradigm shift, from a top-down to a bottom-up approach, in the way electronic devices and sensors will be fabricated in the future. Entities that have been proposed as building blocks for future devices include organic molecules that exhibit rectifying properties and Monolayer Protected Clusters (MPC\u27s) of gold. This research focuses on the development of techniques that will be part of a toolbox to form nanoelectronic devices using MPC\u27s, particularly alkanethiol coated gold nanoclusters. A specific goal of the research is the fabrication of a chemresistive sensor (Nanonose) involving a bilayer of gold clusters with a porphyrin molecule interconnecting the two layers. The molecule acts as the sensing element by changing its conductance, due to a shift in its electronic energy levels, caused by absorption of different chemical species. Several novel processes were developed to facilitate the fabrication of a ‘Nanonose’ structure including the preparation of monolayer arrays of alkanethiolate protected gold clusters at the air-water interface having ordered domains centimeters in diameter, a stamping technique for transferring such an array onto a flat substrate, and a method for patterning such arrays on the micron scale. The MPC arrays were characterized by Transmission Electron Microscopy (TEM), UV-Vis spectroscopy, and Conductive Probe Atomic Force Microscopy (CP-AFM). Electrical characterization of lateral conduction through the arrays was carried out on an interdigitated pattern of gold electrodes that were modified with different molecular layers. The salient features of the results are the reduction in the lateral conductance of the arrays from ca. 1011 Ω/□ to 108 Ω/□ on displacing the alkanethiol coating with conjugated dithiol molecules and the absence of a significant contact resistance between the array and the gold electrode. These results provide a basis for the manufacture of robust nanoelectronic device structures using MPC\u27s

Inkjet printed electroadhesive pads on paper

Astriction between two closely spaced surfaces interacting through fringing electric fields is termed as electroadhesion. Polarisation-induced Coulombic attraction forces at the interface result in electroadhesion. The ability to switch on and off adhesion forces by electrical switching and absence of chemical residues upon detachment are some of the attractive features of electroadhesive devices. To avoid static discharges for ensuring safety and for generating uniform forces, a planar interdigitated pattern on an insulating surface with adjacent fingers being oppositely polarised is the preferred mode of operation for electroadhesive devices. A compliant substrate is required to ensure maximum contact that can lead to practically useful adhesive forces per unit surface area of the electrode. Till now, lithographically-patterned electrodes on polymeric substrates represent the state-of the art in terms of fabricating electroadhesive devices. Herein, we report a low-cost route for fabricating interdigitated electrodes on paper using a desktop inkjet printer, which involves alternate printing of silver salt and potassium bromide/potassium iodide solution, in conjunction with silver halide photographic development process. Typically, electrodes with spacing approximate to 1 mm can be routinely fabricated using a standard office desktop printer. We discovered that in samples where a fixing step was not used to remove excess silver salts, the gaps were reduced further to few hundreds of micron length scales when a high voltage is applied, which can pave the way to higher electric field strength and greater adhesion forces. The results of characterisation of these samples using FESEM, XRD, XPS, sheet resistance and shear load testing will be discussed

Paper-based SERS active substrates on demand

Surface Enhanced Raman Spectroscopy (SERS) is an important technique for molecular analysis under ambient conditions, and the use of paper-based silver nanostructures as a disposable SERS substrate is an attractive option for low-cost, point-of-use analysis. However, the activity of pre-fabricated silver nanostructures is affected by tarnishing and necessitates either storage under inert gas conditions or use of a protective layer on the surface. But neither of these approaches are satisfactory in terms of cost and performance in real-world settings. To address this, we report a print-expose-develop process, based on silver-halide photography, to fabricate SERS active silver nanowire networks on demand using a desktop inkjet printer. This process involves only the printing of silver and halide salt solutions and obviates the need for complex colloidal ink formulation steps reported in earlier studies on inkjet printed SERS substrates. Significantly, the printed and photo-exposed silver halide films retain silver in a stable latent-image form for more than 1 year under ambient conditions. Upon demand at the point-of-use, the latent silver can be easily developed into highly active SERS-active nanostructures, with average EF similar to 10(4), by dipping in a standard photographic developer solution and rinsing

Inkjet-Based Fabrication Process with Control over the Morphology of SERS-Active Silver Nanostructures

Morphological control of silver nanostructures is sought after due to the dependence of optical properties on size and shape. Herein, we report a facile printing process for fabricating silver nanostructured films with wire-like or particle-like morphologies on paper by merely varying the halide composition of precursor salt from potassium bromo-iodide to potassium chloride. Silver was efficiently retained on the top surface of porous paper substrates by printing an excess of potassium halide salt first, leading to more conductive films at lower silver loadings. Furthermore, Raman characterization results show that silver films having nanoparticle morphology have higher SERS activity than samples with nanowire morphology, although the roughness factor of nanowire films is higher than the corresponding nanoparticulate film. Overall, these findings highlight a facile process for controlling the morphology of SERS-active silver nanostructures, which are fabricated on paper using a desktop inkjet printer

Paper swab based SERS detection of non-permitted colourants from dals and vegetables using a portable spectrometer

Rising concern about the use of non-permitted colourants, in common food items such as dals and green vegetables sold in Indian markets, have led to a demand for low-cost point-of-use chemical analysis tools. Conventional food-analysis techniques involving tedious sample preparation protocols are not suited for in-field applications. Surface Enhanced Raman Spectroscopy (SERS) is an analytical technique that is well-suited for point-of-use chemical analysis with molecular level detection capability, which can also serve as a quality assurance tool for businesses. Effective and rapid signal collection from a large-area sample within a field-setting using disposable, low-cost SERS substrates is a key challenge in implementing such a solution. Herein, we demonstrate the use of inkjet-printed thin films comprising of robust nanostructuredsilver as flexible, paper-based SERS (P-SERS) swabs for the direct detection of Metanil Yellow (MY) from toor dal (yellow split pigeon peas) samples and Malachite Green (MG) from green peas and green chillies. The macroscopic uniformity of these thin-films in combination with a portable Raman spectrometer equipped with orbital raster scanning (ORST) technology for signal collection results in an unprecedented precision (RSD similar to 1.6%) upon characterizing samples saturated with Rhodamine-6G (R6G), a standard Raman probe. As several food-cleansing products have appeared in the marketplace, the adulterant removal efficacy of some commercially available `washes' as well as products such as `ozoniser', which was determined by SERS characterization of swabs before and after use, is also reported

Realization of thermally durable close-packed 2D gold nanoparticle arrays using self-assembly and plasma etching

Realization of thermally and chemically durable, ordered gold nanostructures using bottom-up self-assembly techniques are essential for applications in a wide range of areas including catalysis, energy generation, and sensing. Herein, we describe a modular process for realizing uniform arrays of gold nanoparticles, with interparticle spacings of 2 nm and above, by using RF plasma etching to remove ligands from self-assembled arrays of ligand-coated gold nanoparticles. Both nanoscale imaging and macroscale spectroscopic characterization techniques were used to determine the optimal conditions for plasma etching, namely RF power, operating pressure, duration of treatment, and type of gas. We then studied the effect of nanoparticle size, interparticle spacing, and type of substrate on the thermal durability of plasma-treated and untreated nanoparticle arrays. Plasma-treated arrays showed enhanced chemical and thermal durability, on account of the removal of ligands. To illustrate the application potential of the developed process, robust SERS (surface-enhanced Raman scattering) substrates were formed using plasma-treated arrays of silver-coated gold nanoparticles that had a silicon wafer or photopaper as the underlying support. The measured value of the average SERS enhancement factor (2 x 10(5)) was quantitatively reproducible on both silicon and paper substrates. The silicon substrates gave quantitatively reproducible results even after thermal annealing. The paper-based SERS substrate was also used to swab and detect probe molecules deposited on a solid surface

In situ formation of silver nanowire networks on paper

Simple, universally adaptable techniques for fabricating conductive patterns are required to translate laboratory-scale innovations into low-cost solutions for the developing world. Silver nanostructures have emerged as attractive candidates for forming such conductive patterns. We report here the in situ formation of conductive silver-nanowire networks on paper, thereby eliminating the need for either cost-intensive ink formulation or substrate preparation or complex post-deposition sintering steps. Reminiscent of the photographic process of `salt printing', a desktop office printer was used to deposit desired patterns of silver bromide on paper, which were subsequently exposed to light and then immersed in a photographic developer. Percolating silver nanowire networks that conformally coated the paper fibres were formed after 10 min of exposure to light from a commercial halogen lamp. Thus, conductive and patterned films with sheet resistances of the order of 4 Omega/rectangle can be easily formed by combining two widely used processes - inkjet printing and photographic development

Scalable processes for fabricating non-volatile memory devices using self-assembled 2D arrays of gold nanoparticles as charge storage nodes

We propose robust and scalable processes for the fabrication of floating gate devices using ordered arrays of 7 nm size gold nanoparticles as charge storage nodes. The proposed strategy can be readily adapted for fabricating next generation (sub-20 nm node) non-volatile memory devices

Fabrication of electrodes with ultralow platinum loading by RF plasma processing of self-assembled arrays of Au@Pt nanoparticles

Conductive, carbon-free, electrocatalytically active, nanostructured electrodes with ultra-low platinum loading were fabricated using self-assembly of octadecanethiol-coated Au@Pt nanoparticles followed by RF plasma treatment. Bilayer arrays of Au@Pt nanoparticles with platinum loadings of 0.50, 1.04, 1.44, and 1.75 mu g cm(-2) (corresponding to 0.5, 1, 1.5 and 2 atomic layer coverage of platinum on nominally 5 nm gold core) were subjected to RF argon plasma treatment for various durations and their electrical conductivity, morphological evolution, and electrocatalytic activity characterized. Samples with monolayer and above platinum coverages exhibit maximum electrochemically active surface areas values of similar to 100 m(2)/g(pt) and specific activities that are similar to 4x to 6x of a reference platinum nanoparticle bilayer array. The underlying gold core influences the structural evolution of the samples upon RF plasma treatment and leads to the formation of highly active Pt(110) facets on the surface at an optimal plasma treatment duration, which also corresponds to the onset of a sharp decline in lateral sheet resistance. The sample having a two atom thick platinum coating has the highest total mass activity of 48 +/- 3 m(2)/g((pt+au)), corresponding to 44% Pt atom utilization, while also exhibiting enhanced CO tolerance as well as high methanol oxidation reaction and oxygen reduction reaction activity